Cross-Platform Network Applications with Mono

Mono is Ximian's open-source implementation of Microsoft's .NET development framework. .NET contains several different technologies: a set of compilers for many different languages (including Microsoft's new language, C#) that generates platform-independent bytecode; a virtual machine known as the Common Language Runtime (CLR) that runs these bytecodes; and a class library full of useful programs for performing actions ranging from file I/O to GUI creation and operation.

The Mono implementation includes a CLR that runs on Linux, BSD-based systems (including Mac OS X) and Windows, plus compilers for C# and Basic. Mono is a work-in-progress, and many parts of the .NET class library have yet to be implemented, specifically the Windows.Forms group that contains classes for working with the Windows GUI. However, the Mono developers have released bindings for the GTK user-interface toolkit, so cross-platform graphical applications can be constructed even without 100% .NET compatibility. This article describes how to use C#, Mono and Linux to write a useful program, MonoBlog, that can run on any system that runs Mono and GTK. Some familiarity with Glade and C# is assumed, but only at a basic level. Helpful tutorials can be found in the on-line Resources section.

The Mono Web site has instructions for installing the system on Linux, Mac OS X and Windows platforms (see Resources). You also need two additional C# libraries, GTK# and XmlRpcCS. The systems that MonoBlog runs on require the base GTK libraries, which are available on most Linux systems. They probably need to be installed on Windows and Mac OS X systems, however; packages can be found on the GTK Web site (see Resources). Instructions for installing these libraries can be found on their respective Web pages.

MonoBlog is a Weblog editor that can add new posts to a Weblog and edit old ones, as well as provide a way for a user to change configuration settings. Most Weblog systems implement a common base of functionality known as the MetaWeblog API. MonoBlog uses this to communicate with a variety of different Weblog programs, rather than write a separate back end for Movable Type, LiveJournal or Radio Userland systems. The complete C# code for this example is available on the Linux Journal FTP site (see Resources).

Figures 1 and 2 show the user interface of MonoBlog, created using Glade on Linux. The main window in Figure 1 has text controls for entering Weblog titles and the content, plus a series of buttons for updating the Weblog, clearing the forms and quitting the program. The expanse of white on the left-hand side is a GTKTreeView control, which displays a list of older posts the user can click on in order to update. The window shown in Figure 2 is a simple preferences panel where users enter the information that allows MonoBlog to communicate with their Weblogs.

Figure 1. The Main Window

Figure 2. The Preferences Window

One of GTK's useful features is libglade, a library that allows us to construct a program's GUI by reading in the XML files created by Glade, specifying the layout of the widgets in the code itself. The GTK# binding includes this functionality, so building the GUI is quite easy. At the start of MonoBlog, we import the GTK and Glade namespaces with the using statement. Then, in the constructor, we have:

Application.Init();

Glade.XML gxml = new Glade.XML("monoblog.glade",
                               null, null);
gxml.Autoconnect(this);

Application.Run();

The calls to the Application class are required in all GTK programs. Application.Init() performs GTK initialization, and Application.Run() passes control of the program to the GTK main loop, which watches for events and reports signals back when they occur. The standard Glade.XML constructor takes three arguments: a string containing the filename of the Glade file, a string that tells the object the node in the Glade tree where it should start building the interface and, finally, a string that can be used to specify a translation domain for the Glade file in question.

MonoBlog needs to have access to all the nodes in the XML file, both the main window and the preferences panel. No translation is required, so the second and third arguments are null. The Autoconnect() method binds the object given as an argument with the signal handlers and objects defined in the Glade file, allowing that object to respond to events and manipulate widgets. As MonoBlog is a small program, I have contained all the signal handling within the main class. In larger, more complex systems, it might be advisable to separate signal handling out into another class.

To access the widgets, a special declaration is required. The widget must be declared as an instance variable, using a custom attribute:

[Glade.Widget] GTKWidgetType widgetname;